I patched my trac install to supply a conversion from text/x-trac-wiki to text/css, so I can use a certain wiki page ("SiteStyle??") to give my wiki authors direct control over the site CSS (linked from the templates via <link rel="stylesheet" href="/proj/wiki/SiteStyle?format=css" type="text/css" />).
The Trac developers suggested this should be made an extension instead of a core feature (see http://trac.edgewall.org/ticket/5639), but I don't know how to hook a plugin into the respective code or where to start learning. It would be great if someone could pick this up! Perhaps even the aforementioned link element could be inserted by the plugin automatically.